1. Field of the Invention
The present invention relates to a reflective optical sensor and an image forming apparatus and, more particularly, to a reflective optical sensor that detects at least one of the position and the toner density of a toner pattern and an image forming apparatus that includes the reflective optical sensor.
2. Description of the Related Art
Well-known image forming apparatuses that form images using toner include copiers, printers, plotters, facsimile machines, and multifunction printers (MFPs). Such image forming apparatuses form a latent image on the surface of a drum-shaped photosensitive element and develop the latent image into a “toner image” by attaching toner to the latent image.
To form a good toner image, it is necessary to develop the latent image with an appropriate amount of toner. Various development techniques are known, such as a technique using “a two-component-based developer containing toner and carrier” and a technique using “mono toner” in which a developer containing only toner is used. The amount of toner to be supplied to a developing unit for developing the latent image is called “toner density”.
If the toner density is insufficient, because the latent image cannot receive a sufficient amount of toner, an image (output image) with an insufficient density is output from the image forming apparatus. If the toner density is too high, the distribution of the density of the output image shifts toward being high density and an image difficult to recognize is formed. To form a good output image, it is necessary to set the toner density within an appropriate range.
A technique is widely used for adjusting the toner density to within an appropriate range, this technique involving forming a toner-density detection pattern, irradiating the pattern with light (detection light), and determining a change in the intensity of received light (see, for example, Japanese Patent Application Laid-open No. H1-35466, Japanese Patent Application Laid-open No. 2004-21164, Japanese Patent Application Laid-open No. 2002-72612, Japanese Patent No. 4154272, and Japanese Patent No. 4110027).
Conventional sensors that are used to detect the toner density include one or two light-emitting elements or three light-emitting elements, each having different characteristic wave length, and one or two light-receiving elements that receive reflected light. The length of the toner pattern is set to from 15 mm to 25 mm in the main direction so that, even if the position of the toner pattern is incorrect with respect to the sensor, the entire spot of the detection light can illuminate the toner pattern.
With the improvement of color image formation and high speeds in the field of image forming apparatuses, tandem-type image forming apparatuses have become widely used that include a plurality of (four, in general) drum-shaped photosensitive elements.
In such an image forming apparatus, if the positional relation is incorrect between the toner images formed on the photosensitive elements, an output image with a color shift is formed. A technique is widely used for adjusting the positional relation between the toner images, this technique involving forming a position detection pattern, irradiating the pattern with light (detection light), and detecting the position of the pattern using a temporal change in the intensity of reflected light (see, for example, Japanese Patent Application Laid-open No. 2008-276010, and Japanese Patent Application Laid-open No. 2005-238584).
It is noted that during a period when the toner-density detecting process and the pattern-position detecting process are performed, an image forming apparatus cannot perform its primary process, i.e., formation of an image to be output. A toner-density detecting process and a pattern-position detecting process using a conventional reflective optical sensor need a long time to form detection patterns, which reduces the efficiency of the primary process, i.e., formation of an image to be output.
Toner used for detection patterns is so-called “non-contributing toner” because such toner does not contribute to the primary process, i.e., formation of an image to be output. An increase in the amount of the toner used for detection patterns shortens time for replacement of the cartridge containing the toner.